Power supply circuit and RF transponder IC

ABSTRACT

A power supply circuit of the present invention includes: a current detection circuit connected between a first power supply voltage section for applying a first power supply voltage to a first electronic circuit and a second power supply voltage section for applying a second power supply voltage to a second electronic circuit, the current detection circuit having a monitor terminal for monitoring a current flowing from the first power supply voltage section to the second power supply voltage section; and a current compensation circuit connected to the first power supply voltage section and the monitor terminal, the current compensation circuit controlling a compensation current flowing from the first power supply voltage section to a ground based on the monitored current to compensate for current fluctuations caused by load fluctuations of the second electronic circuit.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a power supply circuit, which iscapable of restraining generation of power supply noise and fluctuationsin a power supply voltage by compensating for current fluctuationscaused by load fluctuations of the circuit, and an RF transponder ICcard using such a power supply circuit.

2. Description of the Related Art

In general, an electronic circuit such as an analog circuit or a digitalcircuit is operated by applying a power supply voltage thereto.Fluctuations in a current flowing through the circuit depend on the typeof the circuit. In particular, large current fluctuations occur in adigital circuit such as a CPU, a logic circuit, or a memory. Suchcurrent fluctuations lead to generation of power supply noise orfluctuations in a power supply voltage. The mechanism of generation ofpower supply noise will be described below.

FIG. 11 is a diagram illustrating a structure of a conventionalsemiconductor integrated circuit 3. The semiconductor integrated circuit3 includes a digital circuit 300 and an analog circuit 310. The digitalcircuit 300 includes a CPU 301 and a memory 302. The semiconductorintegrated circuit 3 is externally supplied with a power supply voltageVDD via an inductor L1. The inductor L1 is a parasitic inductor whichoccurs in a conductor line or a bonding wire and has a value of, forexample, several nanofarads. In this case, an internal power supplyvoltage of the semiconductor integrated circuit 3 (chip) is VDD1.

In the case where a current I1 flowing through the semiconductorintegrated circuit 3 (the digital circuit 300 and the analog circuit310) varies as illustrated in FIG. 12B, the internal power supplyvoltage VDD1 of the semiconductor integrated circuit 3 (chip) fluctuatesat the time the current I1 varies, as illustrated in FIG. 12A.

Power supply noise generated in such a manner adversely affects acircuit operation of the analog circuit 310. For example, in the casewhere the analog circuit 310 is a comparator having a hysteresisproperty, a comparison value varies due to fluctuations in the powersupply voltage VDD1, so that malfunction in the analog circuit 310 maybe caused. In the case where the analog circuit 310 is an amplifier,there arises a problem that a power supply fluctuation component isadded to the power supply voltage VDD1 and the quality of a signaloutput from the amplifier deteriorates. Specific examples of malfunctionand property deterioration of an analog circuit will be given below.

FIG. 13 is a graph illustrating input-output characteristics of acomparator having a hysteresis property. In general, a comparator is ina HIGH state when an input voltage is higher than a reference voltage,and the comparator is in a LOW state when the input voltage is lowerthan the reference voltage. In contrast, in a comparator having ahysteresis property, as illustrated in FIG. 13, an output signal Vout isin a HIGH state when an input voltage (an input signal Vin) is equal toor higher than a voltage VH which is higher than a reference voltageVref1 by an offset voltage, and the output signal Vout is in a LOW statewhen the input voltage (the input signal Vin) is equal to or lower thana voltage VL which is lower than the reference voltage Vref1 by theoffset voltage. In the case where an output of the comparator having ahysteresis property is in the HIGH state and a voltage input to thecomparator is equal to the reference voltage Vref1, when the inputvoltage is reduced to be equal to or lower than the voltage VL due togeneration of power voltage noise, the output of the comparator ischanged from the HIGH state to the LOW state and a value of the outputwill not return to the previous value (HIGH state).

In order to solve such a problem, conventionally, a method which usesseparate power supplies for an analog circuit and a digital circuit orprovides a capacitance section having a large capacitance between thepower supply voltage VDD1 section and a ground VSS in the semiconductorintegrated circuit 3 has been used to restrain fluctuations in a powersupply voltage.

However, the above-described methods lead to an increase in the numberof pins (terminals) of the semiconductor integrated circuit 3 (chip) oran increase in area of the capacitance section provided in thesemiconductor integrated circuit 3 (chip). It is preferable that a valueof the capacitance is equal to or more than 1 μF, but a value ofcapacitance which can be provided in the semiconductor integratedcircuit 3 (chip) is about 1 nF at most in the case where a standard CMOSprocess is used, and this is not very effective for great loadfluctuations of the semiconductor integrated circuit 3 (chip).

SUMMARY OF THE INVENTION

A power supply circuit of the present invention includes: a currentdetection circuit connected between a first power supply voltage sectionfor applying a first power supply voltage to a first electronic circuitand a second power supply voltage section for applying a second powersupply voltage to a second electronic circuit, the current detectioncircuit having a monitor terminal for monitoring a current flowing fromthe first power supply voltage section to the second power supplyvoltage section; and a current compensation circuit connected to thefirst power supply voltage section and the monitor terminal, the currentcompensation circuit controlling a compensation current flowing from thefirst power supply voltage section to a ground based on the monitoredcurrent to compensate for current fluctuations caused by loadfluctuations of the second electronic circuit, thereby achieving anobjective of the present invention.

According to above-described structure, the current detection circuit isused to monitor the current flowing through the second power supplyvoltage section to detect current fluctuations caused by loadfluctuations of a digital circuit, etc., and the power supplycompensation circuit is used to compensate for the current fluctuationsto restrain generation of power supply noise and fluctuations in a powersupply voltage, thereby realizing a high-performance power supplycircuit which generates lower noise and can restrain malfunction of acircuit (e.g., an analog circuit) which is liable to be adverselyaffected by the generation of power supply noise and fluctuations in apower supply voltage or a deterioration in the quality of a signaloutput from a circuit (e.g., an analog circuit).

The current detection circuit includes a linear regulator including anoperational amplifier having a noninverting input terminal and an outputterminal, a transistor, and first and second resistances, the transistorhas a source connected to the first power supply voltage section, a gateconnected to the output terminal of the operational amplifier, and adrain connected to the second power supply voltage section, the firstand second resistances are connected in series between the second powersupply voltage section and the ground, and a contact point between thefirst and second resistances is connected to the noninverting inputterminal of the operational amplifier, and the operational amplifiernoninverting input terminal is connected to a reference voltage sectionand the operational amplifier output terminal functions as the monitorterminal.

According to the above-described structure, as described in thefollowing Example 1, it is possible to monitor current fluctuationscaused by load fluctuations of a digital circuit, etc., using the linearregulator.

The current detection circuit includes a third resistance connectedbetween the first power supply voltage section and the second powersupply voltage section, and a monitor terminal on the second powersupply voltage section side.

According to the above-described structure, as described in thefollowing Example 2, it is possible to monitor current fluctuationscaused by load fluctuations of a digital circuit, etc., using thecurrent detection circuit.

The current compensation circuit includes: a subtraction circuit forproducing a difference current between a prescribed current and acurrent controlled by the monitor terminal; and a current circuit forcausing a compensation current proportional to the difference current toflow from the first power supply voltage section to a ground.

According to the above-described structure, as described in thefollowing Example 1, it is possible to compensate for currentfluctuations of a digital circuit, etc., using the power supplycompensation circuit.

The current compensation circuit includes: a reference voltagegeneration circuit for producing a reference voltage; a differentialamplifier circuit for producing a current corresponding to the sum of aprescribed current and a difference current, the difference currentbeing proportional to a value of a difference voltage representing adifference between the reference voltage and a voltage at the monitorterminal; and a current circuit for causing a compensation currentproportional to the sum of the prescribed current and the differencecurrent to flow from the first power supply voltage section to a ground.

According to the above-described structure, as described in thefollowing Example 2, it is possible to compensate for currentfluctuations of a digital circuit, etc., using the power supplycompensation circuit.

The first power supply voltage section is connected to an analogcircuit.

According to the above-described structure, it is possible to restrainmalfunction of an analog circuit which is liable to be adverselyaffected by fluctuations in a power supply voltage or a deterioration inthe quality of a signal output from a circuit.

The second power supply voltage section is connected to a digitalcircuit.

According to the above-described structure, it is possible to detectcurrent fluctuations caused by load fluctuations of a digital circuit,etc., in which large current fluctuations occur.

The current circuit causes the compensation current to flow via a fourthresistance to a ground.

According to the above-described structure, it is possible to restraingeneration of power supply noise by compensating for fluctuations in acurrent flowing through the second power supply voltage section. The sumof the compensation current and the current flowing through the secondpower supply voltage section may not be constant. In this case, powersupply noise to be generated can be reduced by partially compensatingfor current fluctuations.

The sum of a current flowing from the first power supply voltage sectionto the second power supply voltage section and the compensation currentflowing from the first power supply voltage section to a ground isconstant.

According to the above-described structure, it is possible to restrain acurrent which is caused to unnecessarily flow through a current circuitdue to a variance in element properties, etc., of a transistor providedin the current circuit.

An RF transponder IC card of the present invention includes: a coilantenna; and a semiconductor integrated circuit including a tuningcapacitance section having a tuning capacitance, a charging capacitancesection having a charging capacitance, a rectifier circuit, an analogcircuit, a digital circuit, and a power supply circuit which includes acurrent detection circuit connected between a first power supply voltagesection for applying a first power supply voltage to a first electroniccircuit and a second power supply voltage section for applying a secondpower supply voltage to a second electronic circuit, the currentdetection circuit having a monitor terminal for monitoring a currentflowing from the first power supply voltage section to the second powersupply voltage section and a current compensation circuit connected tothe first power supply voltage section and the monitor terminal, thecurrent compensation circuit controlling a compensation current flowingfrom the first power supply voltage section to a ground based on themonitored current to compensate for current fluctuations caused by loadfluctuations of the second electronic circuit. The coil antenna isconnected in parallel with the tuning capacitance section to an input ofthe rectifier circuit, the rectifier circuit has an output connected tothe charging capacitance section and produces the first power supplyvoltage supplied to the analog circuit and the power supply circuitproduces the second power supply voltage supplied from the first powersupply voltage section to the digital circuit, thereby achieving anobjective of the present invention.

According to the above-described structure, it is possible to restrainmalfunction of an analog circuit, etc., caused by load fluctuations in adigital circuit, etc., or the deterioration in the quality of a signaloutput from the analog circuit, thereby realizing a high-performance RFtransponder IC card.

The analog circuit includes a demodulator circuit.

According to the above-described structure, as described in thefollowing Example 3, it is possible to realize a high-performance RFtransponder IC card.

The semiconductor integrated circuit further includes a modulatorcircuit connected in parallel with the tuning capacitance section to aninput of the rectifier circuit.

According to the above-described structure, as described in thefollowing Example 4, it is possible to realize a high-performance RFtransponder IC card having a modulator circuit.

The modulator circuit modulates a circuit impedance using a modulatingsignal.

According to the above-described structure, as described in thefollowing Example 4, it is possible to realize a high-performance RFtransponder IC card having a full-wave rectifier.

The rectifier circuit is a full-wave rectifier circuit.

According to the above-described structure, as described in thefollowing Example 4, it is possible to realize a high-performance RFtransponder IC card having a full-wave rectifier. A half-wave rectifiermay also be used as the rectifier. In such a case, it is possible torealize a high-performance RF transponder IC card having a half-waverectifier.

Thus, the invention described herein makes possible the advantages ofproviding: a high-performance power supply circuit and ahigh-performance RF transponder IC card which can restrain thegeneration of power supply noise and fluctuations in a power supplyvoltage by compensating for current fluctuations caused by loadfluctuations of a digital circuit or the like and which can restrain themalfunction of a circuit (e.g., an analog circuit) which is liable to beadversely affected by the generation of power supply noise andfluctuations in a power supply voltage or a deterioration in the qualityof a signal output from a circuit (e.g., an analog circuit).

These and other advantages of the present invention will become apparentto those skilled in the art upon reading and understanding the followingdetailed description with reference to the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating a structure of a power supply circuitaccording to Example 1 of the present invention.

FIG. 2 is a diagram illustrating a structure of a power supply circuitaccording to Example 2 of the present invention.

FIG. 3 is a diagram illustrating an exemplary structure of a powersupply circuit according to the present invention.

FIG. 4 is a diagram illustrating a structure of another power supplycircuit according to Examples 1 and 2 of the present invention.

FIG. 5 is a diagram illustrating a structure of an RF transponder ICcard according to Example 3 of the present invention.

FIG. 6 is a diagram illustrating a structure of a rectifier circuit inthe RF transponder IC card according to Example 3 of the presentinvention.

FIG. 7 is a graph illustrating a waveform of a power supply voltage AVDDafter an amplitude-modulated signal is rectified in Example 3.

FIG. 8A is a graph illustrating a waveform of a consumption current of adigital circuit in Example 3.

FIG. 8B is a graph illustrating respective waveforms of the power supplyvoltage AVDD in Example 3 with respect to the RF transponder IC cardwith a power supply circuit and the RF transponder IC card without apower supply circuit.

FIG. 8C is a graph illustrating respective waveforms of a demodulatedsignal in Example 3 with respect to the RF transponder IC card with apower supply circuit and the RF transponder IC card without a powersupply circuit.

FIG. 9 is a diagram illustrating a structure of an RF transponder ICcard according to Example 4 of the present invention.

FIG. 10A is a graph illustrating respective waveforms of an impedance ofa coil L2 with respect to the RF transponder IC card with a power supplycircuit and the RF transponder IC card without a power supply circuit inExample 4.

FIG. 10B is a graph illustrating a waveform of a modulating signal inExample 4.

FIG. 10C is a graph illustrating a waveform of a consumption current ofa digital circuit in Example 4.

FIG. 11 is a diagram illustrating a structure of a conventionalsemiconductor integrated circuit.

FIG. 12A is a graph illustrating a waveform of a current I1 flowingthrough the conventional semiconductor integrated circuit.

FIG. 12B is a graph illustrating a waveform of a power supply voltageVDD1 applied to the conventional semiconductor integrated circuit.

FIG. 13 is a graph illustrating input/output characteristic of acomparator having a hysteresis property.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, examples of the present invention will be described withreference to the accompanying drawings. In the drawings described below,components similar to those of the above-described conventionalcomponents will be denoted by the same reference numerals, and thusdetailed explanation thereof will be omitted.

EXAMPLE 1

FIG. 1 is a diagram illustrating a structure of a power supply circuit 1according to Example 1 of the present invention. The power supplycircuit 1 includes a current detection circuit 500 and a currentcompensation circuit 400. A digital circuit 300 is supplied with adigital power supply voltage DVDD as a power supply voltage and ananalog circuit 310 is supplied with an analog power supply voltage AVDDas a power supply voltage.

The current detection circuit 500 includes a linear regulator 200 as amain component. The linear regulator 200 includes resistances R1 and R2,an operational amplifier 210, and a pMOS transistor M1. The transistorM1 has a source connected to an analog power supply voltage AVDDsection, a gate connected to an output terminal of the operationalamplifier 210, and a drain connected to a digital power supply voltageDVDD section. The output terminal of the operational amplifier 210 is amonitor terminal T1 (to which a monitor voltage Va is applied) formonitoring a current flowing through the digital power supply voltageDVDD section (i.e., a current flowing through the digital circuit 300).The resistance R1 is connected to the digital power supply voltage DVDDsection and a noninverting input terminal T2 of the operationalamplifier 210. The resistance R2 is connected to a ground VSS and thenoninverting input terminal T2 of the operational amplifier 210.

The current compensation circuit 400 includes a subtraction circuit 410and a current circuit 420. The subtraction circuit 410 includes aprescribed current source Ic, a transistor M2, and a transistor M3. Thecurrent source Ic is connected to the ground VSS, a drain of thetransistor M2, and a drain of a transistor M3. Each of the transistorsM2 and M3 has a source connected to the analog power supply voltage AVDDsection. The transistor M2 has a gate connected to the monitor terminalT1 (to which a monitor voltage Va is applied) of the linear regulator200. The transistor M3 has a gate connected to the drain thereof to forma control voltage Vc section for the current circuit 420. The currentcircuit 420 includes the transistor M4. The transistor M4 has a sourceconnected to the analog power supply voltage AVDD section, a drainconnected to the ground VSS, and the gate connected to the controlvoltage Vc section.

In the case where a noninverting input terminal T3 of the operationalamplifier 210 is externally supplied with a reference voltage Vref2, thedigital power supply voltage DVDD is represented by:

DVDD=Vref×(R 1+R 2)/R 2  (1).

In this case, when a current flowing through the resistances R1 and R2is set so as to be much lower than a current I1 (a current flowing tothe digital circuit 300), currents I2, I3, and I4 flowing through thetransistors M2, M3, and M4, respectively, are represented by:

I 2=n×I 1  (2)

I 3=Ic−n×I 1  (3),

and

I 4=m×(Ic−n×I 1)  (4).

In the subtraction circuit 410, the difference current I3 between theprescribed current Ic and the current I2 controlled by the monitorterminal T1 (to which a monitor voltage Va is applied) is produced. Inthe current circuit 420, the current I4 proportional to the differencecurrent I3 flows as a compensation current from the analog power supplyvoltage AVDD section to the ground VSS. In the above-describedexpressions, n is a size ratio of the transistor M1 to the transistorM2, and m is a size ratio of the transistor M3 to the transistor M4. I1is a current flowing through the transistor Ml. For example, when gatewidths of the transistors M1, M2, M3, and M4 are widths W1, W2, W3, andW4, respectively, and gate lengths of the transistors M1, M2, M3, and M4are lengths L1, L2, L3, and L4, respectively, n and m are representedby:

n=(W 2×L 1)/(L 2×W 1)  (5),

and

 m=(W 4×L 3)/(L 4×W 3)  (6).

In the expression (4), when the following expressions:

Ic=n×I 0,

and

m=1/n

are given, the current I4 flowing through the transistor M4 isrepresented by:

I 4=I 0−I 1.

In this case, when the current I1 flowing to the digital circuit 300 isincreased by a value of ΔI1 to I1+ΔI1, I4 is I0−I1−ΔI1, and thus acurrent compensating for fluctuations in the current I1 flowing to thedigital circuit 300 flows through the current circuit 420. In FIG. 1, acurrent flowing from the analog power supply voltage AVDD section to theanalog circuit 310 has a value of the sum (I0+Ic) of the current IIflowing to the digital circuit 300, the current Ic flowing through thesubtraction circuit 410, and the current (I0−I1) flowing through thecurrent compensation circuit 420. As a result, except for the currentflowing to the analog circuit 310, the currents flowing from the analogpower supply voltage AVDD section are constant. This means that thenoise generated in the analog power supply voltage AVDD due to loadfluctuations of the digital circuit 300 is reduced.

As can be seen from the description above, in Example 1, it is notnecessary to use separate power supplies for a digital circuit and ananalog circuit for the purpose of restraining fluctuations in a powersupply voltage, thereby reducing the number of pins of a chip. Moreover,it is not necessary to provide a capacitance section having a greatcapacitance in the chip between a digital power supply voltage sectionand the ground for the purpose of restraining fluctuations in a powersupply voltage, thereby preventing an area of a capacitance formed inthe semiconductor integrated circuit (chip) from being increased. As aresult, a high-performance power supply circuit which can be easilyimplemented on a chip and have low power supply noise can be realized.

EXAMPLE 2

FIG. 2 is a diagram illustrating a structure of a power supply circuit 1according to Example 2 of the present invention. The power supplycircuit 1 includes a current detection circuit 500 and a currentcompensation circuit 400. The digital circuit 300 is supplied with thedigital power supply voltage DVDD as a power supply voltage and theanalog circuit 310 is supplied with the analog power supply voltage AVDDas a power supply voltage.

The current detection circuit 500 includes a resistance R10 connectedbetween the analog power supply voltage AVDD section and the digitalpower supply DVDD section and has, on the digital power supply voltageDVDD section side, a monitor terminal T4 (to which a monitor voltage Vais applied) for monitoring a current flowing through the digital powersupply voltage DVDD section (i.e., a current flowing to the digitalcircuit 300).

The current compensation circuit 400 includes a reference voltagegeneration circuit 440, a differential amplifier circuit 430, and acurrent circuit 420. The reference voltage generation circuit 440includes resistances R11 and R12 connected in series between the analogpower supply voltage AVDD section and a ground VSS. The referencevoltage generation circuit 440 outputs a voltage which is produced basedon resistance division between the resistances R11 and R12 as areference voltage Vref3. The differential amplifier circuit 430 includesnMOS transistors M10 and M11, pMOS transistors M12 and M13, and acurrent source I10. The current source I10 is connected to the groundVSS and sources of the transistors M10 and M11. The transistor M10 has adrain connected to a drain of the transistor M12. The transistor M11 hasa drain connected to a drain of the transistor M13. The transistors M12and M13 are separately diode-connected (to have a structure in which asource and a gate are connected to serve as a cathode and a drain servesas an anode). The drain of the transistor M11, and a gate and the drainof the transistor M13 are connected to a control voltage Vc section ofthe current circuit 420. The transistors M12 and M13 have respectivesources connected to the analog power supply voltage AVDD section. Thetransistor M11 has a gate connected to the monitor terminal T4 (to whicha monitor voltage Va is applied). The transistor M10 has a gateconnected to the reference voltage Vref3 section. The current circuit420 includes a transistor M4 which has a source connected to the analogpower supply voltage AVDD section, a drain connected to the ground VSS,and a gate connected to the control voltage Vc section. The currentcircuit 420 controls the control voltage Vc in the differentialamplifier circuit 430 so as to control a current flowing from the analogpower supply voltage AVDD section to the ground VSS.

In the case where a load resistance is Rmax when a maximum current flowsthrough the digital circuit 300, the relationship among the resistancesR10 to R12 and Rmax is represented by:

(R 10/(Rmax+R 10)/2=R 11/(R 11+R 12)  (7).

For example, when respective values of Rmax, R10, R11, and R12 are givenas follows: Rmax=90Ω, R10=10Ω, R11=10 kΩ, and R12=190 kΩ, therelationship between R10 and R11 is given as in R11/R10=1000 for thepurpose of lowering a consumption current.

When the analog power supply voltage AVDD is 3V, the reference voltageVref3 is 2.85V, the power supply voltage DVDD is 2.7V at the maximumdigital load, and the maximum load current of the digital circuit 300 is30 mA. In this case, when a mirror ratio (equivalent to the size ratioof Example 1) of the transistor M13 to the transistor M4 is M andcurrent gain of the differential amplifier circuit 430 is A, a currentI4 flowing through the transistor M4 is represented by:

I 4=M×(A×(DVDD−Vref3)+I 10/2)  (8).

This means that the current I4 flowing through the transistor M4 isproportional to the sum of a current (A×(DVDD−Vref3) which is caused toflow by a difference between the power supply voltage DVDD and thereference voltage Vref3 and a prescribed current I10/2.

In the case where respective values of A, I10, and M are given asfollows: A=0.001, I10=300 μA, and M=100, when the load current flowingto the digital circuit 300 is maximum (i.e., 30 mA), a current flowingthrough the current circuit 420 is approximately 0 mA, and when the loadcurrent flowing to the digital circuit 300 is minimum (i.e., 0 mA), thecurrent flowing through the current circuit 420 is approximately 30 mA.As a result, except for the current flowing from the analog power supplyvoltage AVDD section to the analog circuit 310, the currents flowingthrough the semiconductor integrated circuit 3 are always constant atapproximately 30 mA without being affected by load fluctuations of thedigital circuit 300. The currents flowing through the differentialamplifier circuit 430 and the reference voltage generation circuit 440are very low in comparison to the current flowing through the currentscircuit 420 and the digital circuit 300, and thus is negligible. As aresult, it is possible to restrain generation of noise in the powersupply voltage AVDD section caused by load fluctuations of the digitalcircuit 300, thereby realizing a high-performance power supply circuit.

As can be seen from the description above, in Example 2, it is notnecessary to use separate power supplies for a digital circuit and ananalog circuit for the purpose of restraining fluctuations in a powersupply voltage, thereby reducing the number of pins of a chip. Moreover,it is not necessary to form a capacitance section having a greatcapacitance in the chip between a digital power supply voltage sectionand the ground for the purpose of restraining fluctuations in a powersupply voltage, thereby preventing an area of a capacitance formed inthe semiconductor integrated circuit (chip) from being increased. As aresult, a high-performance power supply circuit which can be easilyimplemented on a chip and have low power supply noise can be realized.

The present invention is not limited to specific examples of the currentdetection circuit 500 and the current compensation circuit 400 describedabove with reference to Examples 1 and 2. As illustrated in FIG. 3, anypower supply circuit falls within the scope of the present invention aslong as the power supply circuit monitors a current flowing through thedigital power supply voltage DVDD section (i.e., a current flowing tothe digital circuit 300) using the current detection circuit 500 andcontrols a compensation current flowing from the analog power supplyvoltage AVDD section to the ground VSS using the current compensationcircuit 400 so as to compensate for current fluctuations of the digitalcircuit 300.

In Example 2, the sum of a compensation current and a current flowingthrough the digital power supply voltage DVDD section is constant, butmay not be constant. Power supply noise to be generated can be reducedby partially compensating for fluctuations in a current flowing throughthe digital power supply voltage DVDD section by the currentcompensation circuit 400.

In Examples 1 and 2, the current circuit 420 has the drain of thetransistor M4 directly connected to the ground VSS, but, as illustratedin FIG. 4, the drain of the transistor M4 may be connected to the groundVSS via the resistance R3. In this case, it is possible to restrain acurrent which is caused to unnecessarily flow through the circuit due toa variance in element properties, etc., of the transistor M4.

Moreover, in Examples 1 and 2, the digital circuit 300 and the analogcircuit 310 are used, but any circuit may be used in place of thedigital circuit 300 as long as the circuit cannot be affected byfluctuations in a power supply voltage, and any circuit can be used inplace of the analog circuit 310 as long as the circuit is easilyaffected by fluctuations in a power supply voltage.

As can be seen from the description above, the present invention ishighly effective in providing a high-performance power supply circuitand is extremely useful.

EXAMPLE 3

FIG. 5 is a diagram illustrating a structure of an RF transponder ICcard 4 according to Example 3 of the present invention. The IC card 4includes a coil antenna L2 and a semiconductor integrated circuit 3. Thesemiconductor integrated circuit 3 includes a tuning capacitance C3section having a tuning capacitance C3, a charging capacitance C4section having a charging capacitance C4, a rectifier circuit 2, ananalog circuit 310, a digital circuit 300, and a power supply circuit 1.The tuning capacitance C3 section and the coil antenna L2 are bothconnected in parallel to an input of the rectifier circuit 2. An outputsection of the rectifier circuit 2 is connected to the chargingcapacitance C4 section. The analog circuit 310 is supplied with a powersupply voltage AVDD as a power supply voltage. The analog circuit 310 isprovided with a demodulator circuit 311 retrieving an input signal froma power supply voltage AVDD section. The power supply circuit 1 producesa power supply voltage DVDD which is supplied from the power supplyvoltage AVDD section to the digital circuit 300. The power supplycircuits described in Examples 1 and 2 can be used as the power supplycircuit 1.

The coil antenna L2 receives an electric power consumed by the RFtransponder IC card 4 and an input signal. The input signal to bereceived by the coil antenna L2 has been amplitude-modulated. Afull-wave rectifier circuit using diodes D1-D4 as illustrated in FIG. 6is used as the rectifier circuit 2. A comparator having a hysteresisproperty is used as the demodulator circuit 311. The input signalreceived by the coil antenna L2 is rectified by the rectifier circuit 2to produce the power supply voltage AVDD having a waveform illustratedin FIG. 7. The demodulator circuit 311 extracts a signal component fromthe power supply voltage AVDD. The demodulator circuit 311 retrieves thepower supply voltage AVDD as a signal from the power supply voltage AVDDsection when a level of the power supply voltage AVDD varies to areference level or higher.

When a current fluctuates in the digital circuit 300 in a manner asillustrated in FIG. 8A, the power supply voltage AVDD has a waveform asrepresented by the dashed line of FIG. 8B. If the RF transponder IC card4 does not have the power supply circuit 1, such current fluctuations(load fluctuations) in the digital circuit 300 cause a malfunction ofthe demodulator circuit 311, so that a demodulated signal output fromthe demodulator circuit 311 has a waveform as represented by the dashedline of FIG. 8C. In contrast, in the RF transponder IC card 4 with thepower supply circuit 1 of Example 3, fluctuations in the power supplyvoltage AVDD caused by the load fluctuations of the digital circuit 300can be restrained so that a demodulated signal output from thedemodulator circuit 311 has a waveform as represented by the solid lineof FIG. 8B. As a result, the demodulator circuit 311 can demodulate asignal without having a malfunction, so that the demodulated signal hasa waveform as represented by the dashed line of FIG. 8C.

As can be seen from the description above, in Example 3, malfunction ofthe analog circuit 310 caused by load fluctuations of the digitalcircuit 300 can be reduced. As a result, a high-performance RFtransponder IC card can be realized.

EXAMPLE 4

FIG. 9 is a diagram illustrating a structure of an RF transponder ICcard 4 according to Example 4 of the present invention. Example 4differs from Example 3 in that the RF transponder IC card 4 according toExample 4 includes a modulator circuit 315 connected to an input of therectifier circuit 2 in parallel with the tuning capacitance C3 section.The modulator circuit 315 modulates an impedance of the coil antenna L2in the semiconductor integrated circuit 3 using a modulating signal.

FIG. 10A is a graph illustrating respective waveforms of an impedance ofthe coil L2 with respect to the RF transponder IC card 4 with a powersupply circuit 1 and the RF transponder IC card 4 without the powersupply circuit 1 when a modulating signal having a waveform asillustrated in FIG. 10B is input to the RF transponder IC card 4. InFIG. 10A, the solid line represents a waveform of the impedance of thecoil L2 of the RF transponder IC card 4 with the power supply circuit 1and the dashed line represents a waveform of the impedance of the coilL2 of the RF transponder IC card 4 without the power supply circuit 1.When a current fluctuates (load fluctuations), as illustrated in FIG.10C, in the digital circuit 300 of the RF transponder IC card withoutthe power supply circuit 1, the impedance of the coil L2 is affected ina manner as represented by the dashed line of FIG. 10A. In contrast, inthe RF transponder IC card 4 with the power supply circuit 1 of Example4, a power supply impedance caused by load fluctuations of the digitalcircuit 300 can be stabilized so that the power supply impedance has awaveform as represented by the solid line of the FIG. 10A.

As can be seen from the description above, in Example 3, malfunction ofthe analog circuit 310 caused by load fluctuations of the digitalcircuit 300 can be reduced. As a result, a high-performance RFtransponder IC card can be realized.

The present invention is not limited to specific examples of thesemiconductor integrated circuit 3 described above with reference toExamples 3 and 4. Any RF transponder IC card falls within the scope ofthe present invention as long as the RF transponder IC card is providedwith a power supply circuit as described in Examples 1 and 2.

In Example 4, the tuning capacitance C3 section is included in thesemiconductor integrated circuit 3, but it may be provided on theoutside of the semiconductor integrated circuit 3 (chip) or a parasiticcapacitance of the rectifier circuit 2 or the coil antenna L2 may beused in place of the tuning capacitance C3 section.

In Examples 3 and 4, a full-wave rectifier circuit is used as therectifier circuit 2, but a half-wave rectifier circuit may be used.

As can be seen from the description above, the present invention ishighly effective in providing a high-performance RF transponder IC cardand is extremely useful.

As described in detail above, according to the present invention, acurrent detection circuit is used to monitor a current flowing through adigital power supply voltage section, etc., to detect currentfluctuations caused by load fluctuations of a digital circuit, etc., anda power supply compensation circuit is used to compensate for thecurrent fluctuations, thereby realizing a high-performance power supplycircuit which generates lower noise.

Moreover, the present invention provides a power supply circuit whichcan realize a high-performance RF transponder IC card having ademodulator circuit and a modulator circuit.

Various other modifications will be apparent to and can be readily madeby those skilled in the art without departing from the scope and spiritof this invention. Accordingly, it is not intended that the scope of theclaims appended hereto be limited to the description as set forthherein, but rather that the claims be broadly construed.

What is claimed is:
 1. A power supply circuit, comprising: a currentdetection circuit connected between a first power supply voltage sectionfor applying a first power supply voltage to a first electronic circuitand a second power supply voltage section for applying a second powersupply voltage to a second electronic circuit, the current detectioncircuit having a monitor terminal for monitoring a current flowing fromthe first power supply voltage section to the second power supplyvoltage section; and a current compensation circuit connected to thefirst power supply voltage section and the monitor terminal, the currentcompensation circuit controlling a compensation current flowing from thefirst power supply voltage section to a ground based on the monitoredcurrent to compensate for current fluctuations caused by loadfluctuations of the second electronic circuit.
 2. A power supply circuitaccording to claim 1, wherein: the current detection circuit includes alinear regulator including an operational amplifier having anoninverting input terminal and an output terminal, a transistor, andfirst and second resistances; the transistor has a source connected tothe first power supply voltage section, a gate connected to the outputterminal of the operational amplifier, and a drain connected to thesecond power supply voltage section; the first and second resistancesare connected in series between the second power supply voltage sectionand the ground, and a contact point between the first and secondresistances is connected to the noninverting input terminal of theoperational amplifier; and the operational amplifier noninverting inputterminal is connected to a reference voltage section and the operationalamplifier output terminal functions as the monitor terminal.
 3. A powersupply circuit according to claim 1, wherein the current detectioncircuit includes a third resistance connected between the first powersupply voltage section and the second power supply voltage section, anda monitor terminal on the second power supply voltage section side.
 4. Apower supply circuit according to claim 1, wherein the currentcompensation circuit includes: a subtraction circuit for producing adifference current between a prescribed current and a current controlledby the monitor terminal; and a current circuit for causing acompensation current proportional to the difference current to flow fromthe first power supply voltage section to a ground.
 5. A power supplycircuit according to claim 1, wherein the current compensation circuitincludes: a reference voltage generation circuit for producing areference voltage; a differential amplifier circuit for producing acurrent corresponding to the sum of a prescribed current and adifference current, the difference current being proportional to a valueof a difference voltage representing a difference between the referencevoltage and a voltage at the monitor terminal; and a current circuit forcausing a compensation current proportional to the sum of the prescribedcurrent and the difference current to flow from the first power supplyvoltage section to a ground.
 6. A power supply circuit according toclaim 1, wherein the first power supply voltage section is connected toan analog circuit.
 7. A power supply circuit according to claim 1,wherein the second power supply voltage section is connected to adigital circuit.
 8. A power supply circuit according to claim 4, whereinthe current circuit causes the compensation current to flow via a fourthresistance to a ground.
 9. A power supply circuit according to claim 5,wherein the current circuit causes the compensation current to flow viathe fourth resistance to the ground.
 10. A power supply circuitaccording to claim 1, wherein the sum of a current flowing from thefirst power supply voltage section to the second power supply voltagesection and the compensation current flowing from the first power supplyvoltage section to a ground is constant.
 11. An RF transponder IC card,comprising: a coil antenna; and a semiconductor integrated circuitincluding a tuning capacitance section having a tuning capacitance, acharging capacitance section having a charging capacitance, a rectifiercircuit, an analog circuit, a digital circuit, and a power supplycircuit which includes a current detection circuit connected between afirst power supply voltage section for applying a first power supplyvoltage to a first electronic circuit and a second power supply voltagesection for applying a second power supply voltage to a secondelectronic circuit, the current detection circuit having a monitorterminal for monitoring a current flowing from the first power supplyvoltage section to the second power supply voltage section and a currentcompensation circuit connected to the first power supply voltage sectionand the monitor terminal, the current compensation circuit controlling acompensation current flowing from the first power supply voltage sectionto a ground based on the monitored current to compensate for currentfluctuations caused by load fluctuations of the second electroniccircuit wherein: the coil antenna is connected in parallel with thetuning capacitance section to an input of the rectifier circuit; therectifier circuit has an output connected to the charging capacitancesection and produces the first power supply voltage supplied to theanalog circuit; and the power supply circuit produces the second powersupply voltage supplied from the first power supply voltage section tothe digital circuit.
 12. An RF transponder IC card according to claim11, wherein the analog circuit includes a demodulator circuit.
 13. An RFtransponder IC card according to claim 11, wherein the semiconductorintegrated circuit further includes a modulator circuit connected inparallel with the tuning capacitance section to an input of therectifier circuit.
 14. An RF transponder IC card according to claim 13,wherein the modulator circuit modulates a circuit impedance using amodulating signal.
 15. An RF transponder IC card according to claim 11,wherein the rectifier circuit is a full-wave rectifier circuit.